Because of the increasing demand from energy storage devices and the earth abundance of sodium relative to lithium, sodium-ion (Na-ion) batteries have been considered as a promising alternative to Li-ion batteries.
It has been a challenge in the Na-ion battery field to improve the rate performance of P2-type layered materials that are layered sodium (Na) transition metal (TM) oxides (NaX(TM)O2) because these layered cathode materials are prone to long-range ordering and phase transitions that hinder performance. Na-vacancy ordering may trap vacancies thereby reducing the Na diffusivity, as has been observed for Li. The flat potential associated with the two-phase region of a first order phase transition causes an electrode to react heterogeneously, thereby degrading the effective rate performance. In addition, phase transitions require local over-potential to nucleate, followed by a strong current concentration once they are nucleated.
The approach of mixing transition metals in layered sodium transition metal oxide materials has been used to attempt to suppress phase transitions by perturbing the ordering of the transition metal sublattice and prevent sodium ions from ordering. A measure of rate performance is the initial discharge capacity of a battery incorporating the layered cathode material when the battery is subjected to galvanostatic testing at a series of rates of charge and discharge.
The main objective in improving the rate performance of layered cathode materials incorporating manganese (Mn) has been to suppress the phase transition known as monoclinic distortion. Monoclinic distortion involves the creation of a two-phase region comprising P2 phase and P′2 monoclinic phase. Monoclinic distortion is an effect of the reduction of Mn4+ to Mn3+ during the discharge of a battery incorporating the layered cathode material during galvanostatic testing.
One higher voltage redox couple that was used to decrease the amount of reduction of Mn4+ to Mn3+ was nickel (Ni), Ni3+ and Ni2+, in the layered cathode material Na0.5(Ni0.23Fe0.13Mn0.63)O2 (P2-MFN). This layered cathode material composition resulted in no phase transition at the end of discharge due to redox compensation by Ni. This material showed a high initial discharge capacity at a low charge/discharge rate. However, this material exhibited poor rate performance.
Another higher voltage redox couple that was used to decrease the amount of reduction of Mn4+ to Mn3+ was cobalt (Co) Co3+ and Co2+, in the layered cathode material Na2/3(Mn1/3Fe1/3Co1/3)O2 (P2-MFC1/3). Although this cathode material did not have a phase transition at the end of discharge, the Co redox couple had a limited initial capacity in the high voltage region, at ˜126 mAh/g for an upper voltage cutoff of 4.1 V and rate C/10 (10 mA/g). This material also exhibited poor rate performance, with an initial discharge capacity of ˜78 mAh/g at 1 C rate (100 mA/g), about 60% of the C/10 rate capacity, at with a large increase in polarization at higher rates. At an upper voltage cutoff of 4.3 V, the irreversible capacity increased to 31 mAh/g from ˜15 mAh/g for an upper voltage cutoff of 4.1 V.
Layered cathode materials with better rate performance than the aforementioned Mn-containing materials included Na(Fe1/2Co1/2)O2 (O3-FC) and carbon-coated NaCrO2 (O3-CCr). These materials had slower declines in performance with rate than did the Mn-containing materials, but had significantly lower initial discharge capacities at low charge/discharge rates than did P2-MFN. Thus, improvements in performance of such materials are needed.